Model-based assessment of muscle forces and strain distribution in the femur of Cabot’s Tragopans (Tragopan caboti)

The hindlimbs play a crucial role in bird locomotion, making the biomechanical properties of the musculoskeletal system in these limbs a focal point for researchers studying avian behaviour. However, a comprehensive analysis of the mechanical performance within the long bones of hindlimbs during locomotion remains lacking. In the present study, the strain and deformation of the femur of Cabot’s Tragopans (Tragopan caboti) were estimated. We employed inverse simulation to calculate the force and moment of femoral muscles during mid-stance terrestrial locomotion and conducted finite element analysis to calculate femoral strain. Results showed that during mid-stance, the femur experiences combined deformation primarily characterized by torsion, bending, and compression. It emphasises the importance of considering the influence of varying loads on bone adaptation when investigating bone form-function relationships. Muscles were found to play a significant role in offsetting joint loads on the femur, subsequently reducing the deformation and overall strain on the bone. This reduction enhances femoral safety during locomotion, allowing birds to meet mechanical demands while maintaining a lightweight bone structure. Notably, the M. iliotrochantericus caudalis significantly reduces torsional deformation of the proximal femur, protecting the vulnerable femoral neck from high fracture risk induced by rotation load. Given that the femur torsion during terrestrial locomotion in birds is associated with changes in hindlimb posture due to their adaptation to flight, the characteristics of M. iliotrochantericus caudalis may provide insight into the locomotor evolution of theropods and the origin of avian flight.